linux 系统设置ipv4与ipv6出口优先级
ipv4优先：

sed -i 's/#precedence ::ffff:0:0\/96  100/precedence ::ffff:0:0\/96  100/' /etc/gai.conf
curl ip.sb

ipv6优先：

sed -i 's/precedence ::ffff:0:0\/96  100/#precedence ::ffff:0:0\/96  100/' /etc/gai.conf
curl ip.sb

#常用命令